As is known in the art, large host computers and servers (collectively referred to herein as “host computer/servers”) require large capacity data storage systems. These large computer/servers generally includes data processors, which perform many operations on data introduced to the host computer/server through peripherals including the data storage system. The results of these operations are output to peripherals, including the storage system.
One type of data storage system is a magnetic disk storage system. Here a bank of disk drives and the host computer/server are coupled together through an interface. The interface includes “front end” or host computer/server controllers (or directors) and “back-end” or disk controllers (or directors). The interface operates the controllers (or directors) in such a way that they are transparent to the host computer/server. That is, data is stored in, and retrieved from, the bank of disk drives in such a way that the host computer/server merely thinks it is operating with its own local disk drive. One such system is described in U.S. Pat. No. 5,206,939, entitled “System and Method for Disk Mapping and Data Retrieval”, inventors Moshe Yanai, Natan Vishlitzky, Bruno Alterescu and Daniel Castel, issued Apr. 27, 1993, and assigned to the same assignee as the present invention.
As described in such U.S. patent, the interface may also include, in addition to the host computer/server controllers (or directors) and disk controllers (or directors), addressable cache memories. The cache memory is a semiconductor memory and is provided to rapidly store data from the host computer/server before storage in the disk drives, and, on the other hand, store data from the disk drives prior to being sent to the host computer/server. The cache memory being a semiconductor memory, as distinguished from a magnetic memory as in the case of the disk drives, is much faster than the disk drives in reading and writing data. The cache memory thus provides a global memory for the system.
As is also known in the art, some protocols used by the host computer/server, such as ECSON and FICON are byte-oriented. That is, each I/O data to be transferred (i.e., an I/O transfer) is made up of one or more time-separated packets. Thus, there may be a requirement to assemble contiguous word-aligned data streams based on scatter/gather descriptor lists which describe byte-oriented data. That is, in some protocols, the data is fed to the interface in discrete, time separated, packets with each packet being stored in different sections of a shared memory included in the director. When all packets associated with a particular I/O data to be transferred transfer are collected, the entire I/O is then transferred to the global memory.